Stainless steel



United States Patent 3,253,966 STAINLESS STEEL Frank A. Malagari, Jr.,Freeport, and Adolph J. Lena,

This invention relates to stainless steel having a critical range ofcomposition which is characterized by exhibiting a high degree ofnondirectionality when utilized in its cold worked condition.

The ever-expanding market for stainless steels has witnessed the use ofthese steels in various structural components, especially Where a highdegree of corrosion resistance is necessary together with the ability towithstand cyclical loads. In general, it is known that stainless steelsin use today are those referred to as the austenitic stainless steelsand those steels referred to as ferritic or martensitic and varioushybrids thereof. Typical of the former class is the group of steelsdesignated as the AISI Type 300 series, for example, Type 301. ample ofthe latter class is the martensitic stainless steels typified by theAISI Type 410 composition or the territic steels as typified by the AISIType 430 composition. Each of these various types of compositions isdependent on the chemical composition for the microstructure exhibitedby the steel and the microstructure in turn will have an influence onthe mechanical properties exhibited by the steel. It is also well knownthat most of the austenitic-type stainless steels are stabilized to sucha degree that they will not transform from the metastable austenite tomartensite with a consequent increase in strength. Therefore, the onlypractical means of strengthening these types of austenitic stainlesssteels, as distinguished from those of the transformation hardeningtype, for example, those steels known as the semi-austenitic stainlesssteels, an example of which is AM-355, is dependent upon an operationwhich will work harden the steel to obtain a designated temper tothereby increase their strength characteristics. Typically, AISI Type301, an austenitic stainless steel, is work hardened and marketedcarrying a temper designation of A hard, /2 hard, full hard, extra fullhard, etc. These temper designations Anex-- refer to a degree of coldworking which has been applied to the steel and, consequently, alsorelate to the strength level exhibited by the steel to which the varioustemper designations are applied. These steels as thus marketed,characteristically exhibit excellent corrosion resistance and strengthcharacteristics, thus making them useful in a myriad of applicationsincluding use as structural components. i

It is to be noted, however, that there is one serious "ice of stainlesssteel in an application which is subjected to a cyclical loadinginvolving both tension and compression, or when used under compressiveloading alone, the design criteria must be based on the longitudinalcompressive yield strength. The yield strength, when measuredtransversely to the direction of cold working, does not show any greatdivergence whether such yield strength is measured in tension orcompression. The hiatus which occurs between the tensile yield strengthand the compressive yield strength, each measured in the longitudinaldirection, can seriously detract from the use of this steel.

The stainless steel of the present invention, with its controlledcompositional range, produces a close correlation between the tensileyield strength and the compressive yield strength, each measured in thelongitudinal direction after the steel lias been cold worked to effect areduction in the cross sectional area of between about 10% and about40%. The steel, as thus cold worked, will exhibit a ratio of compressiveyield strength to tensile strength, each measured in the longitudinaldirection, of greater than about .8.

An object of the present invention is to provide a stainless steel whichis suitable for use in application requiring corrosion resistance andstrength.

Another object of the present invention is to provide a stainless steelwhich, after cold working up to 40%, will have a close correlationbetween the tensile yield strength andcompressive yield strength wheneach is measured in the longitudinal direction.

Another object of the present invention is to provide a stainless steelhaving a critically controlled composition and which is subjected tocold working in order to increase the strength characteristics of thesteel without imparting a high degree of directionality thereto.

A more specific object of the present invention is to provide astainless steel having a controlled chemical composition and which,after being subjected to a reduction in the cross sectional area ofbetween about 10% and 40%, will exhibit a ratio of compressive yieldstrength to tensile yield strength measured in the longitudinaldirection of greater than .8.

Other objects of the present invention will become apparent to oneskilled in the art when taken in conjunction with the followingdescription and claims.

The steel of the present invention contemplates a critical composition.This critical composition requires carbon within the range between 0.08%and 0.12%, up to about 1.25% manganese, up to about 1% silicon, fromabout 15.3% to about 16.6% chromium, from about 5.5% to about 6.2%nickel, from about 0.025% to about 0.06% nitrogen, and the balancesubstatnially iron with incidental impurities. Each of the elementsperforms the usual function normally associated wit-h'the element inaustenitic-type stainless steels. In this respect, the balancing of thecarbon and nitrogen and nickel is exceedingly critical as respects thestability of the microstructure of the steel. In this same vein, it isnoted that the chromium content increases the stability of the steel andthereby is influential in decreasing the work hardening rate therebyaffecting the sluggishness of the austenite to martensitetransformation. This occurs despite the fact that chromium is a strongferrite-forming element and the use thereof requires a criticalbalancing of the austenite-forming elements such as carbon, nitrogen andnickel.

It is also noted that molybdenum cannot be used as a direct substitutefor chromium on a 1:1 ratio basis in the steel of the present invention.While molybdenum, a strong ferrite-forming element, is effective forincreasing the yield strength and the hardness of the steel in theannealed condition, nonetheless, it is apparently effective when used ina 1:1 ratio for decreasing the stability of the steel, thus permittingthe steel to transform to martensite With a consequential effect thatthe steel exhibits a high degree ofdirectionality. While nickelmaterially contributes to the stability of the alloy, it must be limitedtogether with the carbon and nitrogen in order to maintain the properdegree of balance.

In order to more clearly define the limits of the steel of the presentinvention reference may be had to Table I which tabulates the generaland optimum ranges of composit-ion.

TABLE I.OHEMICAL COMPOSITION (PERCENT BY WT.)

Element General Range Optimum Range About 0.1. .0.

As was stated hereinbefore, AISI Type 300 series stainless steels, forexample, Type 301, derive enhanced mechanical properties through theapplication of specific amounts of cold Work. It has been postulatedthat cold Work effects a Work hardening of these stainless steels, thestrengthening occurring through the work hardening of the face centeredcubic structure of the austenite, the transformation of austenite tomartensite and the further work hardening of the martensite. The amountof transformation which can occur as a result of cold working isdependent upon the chemical composition and should be greatest in thoseaustenitic stainless steels which contain the lowest amount of alloyingcomponents. Thus, it has been observed that for the AISI Type 300stainless steels, Type 301, which contains the lowest amount of alloyingcomponents and which has a nominal composition of about 17% chromium and7% nickel, exhibits the highest work hardening rate and, in addition,this steel also exhibits a transformation product when sufficiently coldworked. This steel can be cold worked to develop desirable highstrengths for structuralapplications. Where the amount of the alloyingelements increases, the austenite may attain such a high degree ofstability that the only strengthening possible is the work hardening ofthe face centered cubic lattice structure of the austenite. Further coldworking is insufficient to impart sufiicient energy to transform themetastable austenite to martensite. As noted previously, AISI Type 301stainless steel in the cold worked condition exhibits a high degree ofdirectionality when these steels are tested in the direction which islongitudinal to the direction of the cold working. In particular, thisdirectionality is manifested by a lower compressive yield strength thanthe tensile yield strength when measured in the longitudinal direction.This discrepancy, however, is not noted when the same cold worked steelis tested in tension and in compression in a direction transverse to thedirection of cold working. Thus, in Type 301 having a temper designationof full hard, the longitudinal tensile yield strength may be 140K s.i.but the .taining less than 50% ferromagnetic component.

longitudinal compressive yield strength may only be K s.i., yet thetransverse tensile and compressive yield strengths are equivalent and ofa magnitude similar to the longitudinal tensile yield strength. Whilethe directionality of this cold worked austenitic stainless steel can beminimized or eliminated in some instances by a sub-critical heattreatment, other adverse effects are noted in the measured mechanicalproperties of the steel when so-heat treated. This is particularly truewith respect to the ductility exhibited by the alloy. While the steel ofthe present invention contemplates a usage where strength criteria areof paramount importance, it becomes necessary for the steel to exhibit ayield strength measured in the longitudinal direction both in tensionand compression of K s.i. minimum after the steel has been cold workedsufficiently to effect a reduction in the cross sectional area rangingbetween about 30% and about 40%.

In order to more clearly illustrate the critical nature of the steel ofthe present invention, reference is specifically directed to Table IIwhich sets forth the chemical composition of a series of steels whichwere made and tested to illustrate the critical nature of the chemicalcom position.

TABLE II.-OHEMICAL ANALYSES Heat 0 Mn Si Cr Ni N Other 10 99 64 16. 36.0 03 .09 .85 .62 16.2 5.4 .04 10 83 60 16.6 5. 8 .04 12 77 53 16.3 5.5 04 16 82 61 16.1 5. 4 04 19 .84 56 16.3 5. 4 04 09 78 57 15. 6 6.0 04.10 .77 54 15.8 6.0 .04 10 82 60 16.8 6.0 05 10 85 67 17. 9 5. 8 04 0987 61 14. 6 6.2 04 1.0 M0. 09 86 56 13. 4 6. 1 04 1.6 M0. 11 88 58 12. 46. 2 04 2.0 M0. 09 83 60 11.5 6. 2 04 2.6 M0. 10 82 56 16.2 5. 5 O5 1182 61 16.7 6. 4 .04 10 80 .69 16. 7 6. 8 04 08 97 72 16. 5 6. l 03 09 9367 15. 6 5. 5 05 1O .90 69 15.3 6.0 .06 10 99 73 16, 4 6. 2 03 09 91 .6816.6 5. 5 03 are set forth hereinafter in Table III.

It is to be noted in Table III that one column is headed percent Mag.Res. This refers to the percent of magnetic response exhibited 'by eachof the steels set forth in Table III as measured by the Magne-gage. Asis well known, the Magne-gage measures the amount of ferromagneticcomponent in the steel and its accuracy is limited to a steel having anaustenitic structure and con- Consequently, in Table III where themagnetic response is set forth therein as 50 it will be understood thatthis includes amounts greater than 50, since the magnetic response asmeasured by the Magne-gage is only accurate where up to a 50% maximumferromagnetic component is present within the steel.

TABLE III.EFFECT OF COLD WORK ON MECHANICAL PRPERTIESContinued TensionCompres- Percent Percent sion, C.Y.S. Heat Cold Hardness Mag. 2% Y.S.T.Y.S.

Reduction Res. 2% Y S Ultimate Percent (K. s.i.)

(K. s 1 Str. (K. s.i.) El. 2"

FL-95 85 R 43 38. 2 152. 2 33. 39. 5 1. 03 5 38 R0--- 50 94. 2 190. 417. 5 91. 3 96 FL-88 0 27.5 R 50 37. 2 192. 2 19.0 53. 8 1. 44 5 44.5 R50 153. 5 216.0 11.5 170. 2 1.10

10 46 Ru- 50 150. 9 218. 7 11. 5 166. 0 l. 09

FO-49 0 88.5 R 28-34 40. 3 166. 4 33. 5 48. 3 1. l9 5 43 Ru- 50 115. 7210. 7 19. 0 113. 3 97 20 47.5 RP... 50 176.0 227. 2 l3. 0 158. 3 89FL-96 0 86.5 R 4. 2 33. 7 152. 0 35.0 39.2 1. 16 5 38.5 Ba... 50 84.3195. 9 21. 5 84. 8 1. 00

Fla-99 0 92 Rb--- 36 39. '4 174. 4 36. 0 43. 5 1.10 5 42 Re. 50 100. 1210. 9 18. 5 109. 9 1.09

From the test results recorded in Table III for Heats FC-4, FE-26,FB-95, FB-93 and FB-92, it is apparent that increasing the carboncontent is highly effective for stabilizing the austenite of the steel.Thus, in the annealed condition, while Heat FC-4 appears to be magneticfor work hardening the martensite a sufiicient degree to induce highlydirectional mechanical properties. This is substantiated by the factthat the Magne-gage readings, which only measure up to 50% ferromagneticcomponent, indicate a greater than 50% magnetic response throughout theworking history of the heat. A comparison of the mechanical propertiesillustrates that with as little as 10% cold reduction, the ratio of thecompressive yield strength to the tensile yield strength (each measuredin the longitudinal direction ofthe cold reduction) effectively dropsbelow .8 thus indicating the highly directional nature of the steel.While it appears that substantially all of the alloying elements exceptthe nickel are within the range set forth in Table I it is clear thatthis deficiency in the nickel content in Heat FC-4, is suflicient tomake the steel sufficiently unstable that the cold working is effectivefor inducing a high degree of directionality to this steel.

Reference is directed to Heat FA-22 which has a composition within thelimits of the preferred range as set forth hereinbefore in Table I. Thissteel, in the annealed condition, exhibits a 40% magnetic response asmeasured by the Magne-gage and a ratio of compressive yield strength totensile yield strength of greater than about .8. Cold working thesubject steel various amounts up to about 40% is effective fortransforming additional amounts of austenite to martensite so that theMagne-gage readings indicated a greater than 50% magnetic response aftercold working only 5%. When this steel was tested both in tension andcompression, it was noted that throughout the working range, i.e. upuntil 40% cold work was performed on the steel, the steel exhibited aratio of compressive yield strength to tensile yield strength of greaterthan about .8 when measured in the longitudinal direction of coldworking. While the steel had this excellent non-directionality, it alsoexhibited a compressive yield strength of about 209.9K s.i. and atensile yield strength of about 241.9K s.i., after a 40% reduction inthe cross sectional area thus indicating the high level of mechanicalproperties exhibited by this steel through the close control of thechemical composition, to be explained more fully hereinafter.

The magnetic saturation level in these steels is a function of coldwork. Consequently, the magnetic saturation is indicative of the amountof the ferromagnetic component present within the steel. When themagnetic saturation was measured in the annealed condition on Heat FC-4preparatory to any cold working, the steel had a saturation induction ofabout 500 gausses at a magnetization level in the range between 200 and4000 oersted. After cold reduction as much as 40% saturation inductionmerely increased to a value of less than about 13,000 gausses. Thus, thelow slope of the curve clearly illustrates very small change in theferromagnetic component of this steel. This further substantiates theproposition that substantially all of the cold work performed on thissteel was effective for work hardening the martensitic constituent.

Reference to Heat FE26, which has a composition within the limits setforth in Table I, clearly illustrates the structure sensitivity of thesteel. Thus, the steel had an initial magnetic response of about 25.6%and it required a working of up to about 10% in order to obtain amagnetic response of greater than 50%. Magnetic saturation dataillustrated a low magnetic saturation in the annealed condition whichprogressively increased with the cold reduction. This increase continueduntil about 40% reduction was applied to the steel after which themagnetic saturation showed little increase. Measurement of the tensileproperties on this heat which are set forth in Table III, clearlyillustrate that cold working within the range between about 10% andabout 40%, is effective for increasing the mechanical properties withoutadversely affecting the ratio of the compressive yield strength to thetensile yield strength. This ratio was maintained at greater than .8throughout the working range and clearly illustrates that with as muchas 30% cold reduction extremely high yield strengths are obtained whenmeasured both in compression and tension. These yield strengths areaccomplished by adequate ductilities of about 8.5%. Thus the increase inthe nickel and the slight increase in the carbon content was effectivefor controlling the rate of the austenite to martensite transformationand the subsequent work-hardening of the martensitic phase withoutinducing highly directional characteristics to the excellent level ofmechanical properties exhibited by the steel.

Heat FB-95 also illustrates the critical nature of the chemicalcomposition of the steel of the present invention. The chemical analysisof this steel shows a carbon content of 0.12% and a nickel content of5.5 such carbon content being at the upper limit and the nickel contentat about the lower limit. In the annealed condition, this steelexhibited about a 20.2% magnetic response when measured on theMagne-gag'e. After the application of as little as cold work, theferromagnetic component of the steel increased sufficiently so that theMagne-gage readings indicated a greater than 50% response. This steelwas cold worked various amounts to effect a reduction in the crosssectional area of up to 40%. This cold working was not effective forconferring directionality to the steel since the ratio of thecompressive yield strength to the tensile yield strength, each measuredin the longitudinal direction, was greater than about .8. Thus, it isclear that the steel of this composition is on the borderline andclearly demonstrates the critical nature of the chemical composition.Increasing the carbon content to 0.16% and 0.19% as in Heats FB-93 andFB92, clearly illustrates the effect of the carbon in stabilizing thesteel. Thus, from Magne-gage readings it is apparent that it requiresreductions of greater than in order to obtain greater than a 50%response for Heat PB93. Heat FB- 92 requires greater than cold reductionin order to attain a greater than 50% magnetic response. The mechanicalproperties set forth in Table III clearly illustrate the highlydirectional nature of these steels. Magnetic saturation measurementsalso' effectively illustrate the stability of the steel. Thus, it isclear that with a small variation in the carbon content the stability ofthe steel can be changed sufiiciently so that the steel will exhiibt ahigh degree of directionality when cold worked up to about 40%. Thisindicates that the chemical composition of the steel of the presentinvention must be critically controlled in order to obtain a steelhaving a ratio of the compressive yield strength to the tensile yieldstrength, each measured in the longitudinal direction of greater thanabout .8. Variation in the carbon content which affects the stability ofthe steel, also affects the rate of work-hardening so that carboncontents outside the limits set forth in Table I affect thetransformation of the austenite to martensite during cold working withthe result that the steel will exhibit directional properties. Thecurves from the magnetic saturation data confirm this.

The effect of a variation in the chromium content is illustrated byHeats FE25, FD-Sl, FD-86 and FE-30. These heats have a chromium contentvarying between 15.6% and about 17.9%. From the test results recorded inTable III it is clear that both Heats FE- and FD81 show non-directionalcharacteristics when they are cold worked up to about The magneticresponse measurements indicate that Heat FE-25 has a greater than 50%magnetic response in the annealed condition. Increasing the chromium-contenta strong ferrite-forming elementis effective for increasing theaustenite stability and, as a result thereof, the steel of Heat FD-Sl inthe annealed condition shows a magnetic response. When these same heatsare tested for their mechanical properties it is apparent that up to a40% cold reduction can be applied to these heats, while stillmaintaining a ratio compressive yield strength to tensile yield strengthof greater than about .8. Thus, through the controlled chemicalcomposition of these heats, it being noted that all of the alloyingelements are within the range set forth in Table I, an outstandingcombination of mechanical properties is readily obtained which exhibitthe desired degree of non-directionality. Further increases of thechromium content as in Heats FD-86 and FE-30 are effective forstabilizing the steel so that a greater than 10% cold reduction isnecessary in order to obt ain'a greater than magnetic response asmeasured by the 10 Magne-gage. Magnetic saturation measurements clearlyindicate that the saturation induction decreases with increasingchromium and the steels apparently have not attained the highest degreeof saturation induction with cold working up to 40%. Consequently, it isapparent that the steels FD-86 and FE-30 have an insufficient amount oftransformation product. As a result thereof these steels showconsiderable directionality which is apparently caused by the stabilityof the work hardened austenite through the addition of these higheramounts of chromium. Thus, the ratio of the compressive yield strengthto the tensile yield strength decreases below about .8 with the resultthat the steels show an increase in the directionality of the propertieswhen the composition thereof is increased as respects the chromiumcontent to a value in excess of about 16.6%. Replacement of part of thechromium with molybdenum (and in this respect, chromium contentspartially substituted by molybdenum, show substantially similar resultsas will be set forth hereinafter.

Reference is now directed to Heats FB-90, FB-88, FB-87 and FB-86, inwhich the compositional dependence of the partially substituted chromiumcontent is illustrated in this series of steels. The chromium contentwas varied through substitution of molybdenum, the substitution being ina ratio of about 1:1. At first glance, it is apparent that the effect ofmolybdenum in each of ,these steels is to provide steel with a highdegree of instability thereby resulting in an early transformation ofaustenite to martensite with the result that, effectively, asingle-phase alloy is subjected to the cold working and thus exhibits ahigh degree of directionality. Thus, after a cold reduction of up to10%, these steels exhibit highly directional characteristics wherein thecompressive yield strength to tensile yield strength ratio is less thanabout .8. The level of the yield strength in the annealed condition ofeach of these steels is high which is accompanied by high hardness.Magnetic sauration measurements reveal that increasing the molybdenumcontent at the expense of the chromium, results in a successively highersaturation induction of these steels in the annealed condition.Moreover, the slope of the curve with respect to cold reductionsindicates that very little transformation is taking place. Consequently,all of the cold working is apparently accomplished on the substantiallysingle-phase steels (probably martensitic in character), resulting in ahigh degree of directionality being imparted thereto. The data set forthin Table III indicates thatincreasing the molybdenum content alsopromotes higher room temperature strengths with increasing cold work.From these data it appears that molybdenum cannot be substituted for aportion of the chromium on a 1:1 basis at this level of chromium.

Heats FD-82, FD-lOO and FE-29 clearly show the effect of nickel on thesteel of the present invention. Thus, an increase in the nickel contentof from about 5.5% up to about 6.8% clearly illustrates the adverseeffect on the directionality of the properties of these steels which isproduced by increasing the nickel content to an amount in excess ofabout 6.2%. Thus, Heat FD-82 having a nickel content of about 5.5 andwhich had a 40% magnetic response in the annealed condition at roomtemperature, had a ratio of compressive yield strength to tensile yieldstrength in excess of about .8 after cold working up to 40%. On theother hand, Heat FD-lOO which had a nickel content of 6.4% and HeatFE-29 which had a nickel content of 6.8%, indicate that these amounts ofnickel are effective for conferring a high degree of stability to thesteel. This is shown by the percent magnetic response and was confirmedby the magnetic saturation measurements. Thus, the test results setforth in Table III clearly illustrate that increasing the nickel contentto more than about 6.2% is effec tive for inducing a high amount ofdirectionality to the 1 1 steel. With as little as 5% cold reduction,these high nickel-containing heats will usually have a ratio of compressive yield strength to tensile yield strength of less than .8.

The test results set forth in Table HI for Heat FL-95 clearly illustratethe effect of the extreme ends of the range as respects the carbon andnitrogen contents. It is noted that the nickel and chromium contents arenear the upper limit in this steel in order to obtain the proper degreeof balance in the chemical composition. As annealed, the steel had amagnetic response of 43% as measured by the Magne-gage and was in arelatively soft condition, the hardness being measured at 85 R Thissteel after cold working up to 40%, had a compressive yield strength totensile yield strength ratio of greater than .8. Magnetic saturationmeasurements indicated a low saturation of the steel prior to coldworking; thereafter, the slope of the curve indicates an initial rapidrise in the magnetic saturation followed by decrease in the rate ofsaturation, yet the level increases for cold reductions of up to about40%. Thus, it is clear that at least 0.08% carbon and 0.025% nitrogenare necessary in the steel of the present invention in order to obtainan adequate strength level without adversely affecting the ratio of thecompressive yield strength to tensile yield strength.

Heat FL-88 clearly shows the effect of chromium and nickel near thelower limit where the carbon and nitrogen are near the mid-point of therange set forth in Table I. Cold working the steel of Heat FL-88 toeffect re-' ductions of up to about 40%, increased the strength levelsyet the ratio of compressive to tensile yield strengths exceeds .8.While the initial magnetic saturation was higher than that of Heat FL-95as would be expected since Magne-gage readings indicated Heat FL-88possessed a greater than 50% magnetic response, yet the slope and theshape of the curves are quite similar, the only difference being adisplacement upwardly for the latter heat as compared to Heat FL-95.

Reference to the test results recorded in Table III for Heat FO-49clearly illustrates the effect of chromium near the lower limit of theGeneral Range. Thus, with a chromium content of about 15.3% it is notedthat cold reductions of up to about 40% are effective for showing anoutstanding increase in the attainable mechanical properties and withoutany directionality occurring in this steel. The compressive yieldstrength to tensile yield strength ratio exceeds .8 and magneticsaturation measurements indicate :a curve substantially similar to thatfor Heats FL-95 and FL-88.

Heat FL-96 illustrates the effect of a nickel content of 6.2% whichapproaches the upper limit of the nickel range. This steel showed aninitial magnetic response of 4.2% tested with a Magne-gage. Magneticsaturation measurements when plotted against the percent cold reductionindicate that for a small amount of cold work there is a fast rise inthe magnetic saturation. This would correspond to the amount offerromagnetic material present and indicates the relative degree ofstability of the steel. Thus, as would be expected, while this steel isslightly more stable than that of Heat FL-95 and FL- 88, it quicklytransforms to martensite with small amounts of cold work as shown by theslope of the magnetic saturation curve as well as the percentagemagnetic response measured by the Magne-gage readings. Cold working upto 40% is effective for increasing the level of the mechanicalproperties and does not adversely affect the non-directionality of thesteel, it being noted that this steel had a compressive yield strengthto tensile yield strength ratio of greater than .8 after as much as 40%cold reduction. Thus, it would appear that the nickel content should notexceed about 6.2% in the steel of the present invention.

Heat FL-99 which contains a chromium content at the upper limit, anickelcontent near the lower limit and a nitrogen content near the lowerlimit, also favorably responds to produce outstanding mechanicalproperties which are non-directional. While this steel had an initialmagnetic response of 36% as measured by the Magnegage, cold working aslittle as 5% was sufficient to provide a greater than 50% response asmeasured by the Magnegage. Magnetic saturation measurements follow thesame general pattern as that for Heats FL9-5 and FL-88. After coldworking up to 40% the compressive yield strength to tensile yieldstrength ratio exceeded .8, thus illustrating the non-directionality ofthis steel.

From the foregoing, it is apparent that the steel of the presentinvention must be critically controlled within the limits set forthhereinbefore in Table I. Variations from the ranges set forth in Table Iapparently cause a variation in the stability of the steel which willaffect the rate of transformation as well as the rate of work-hardeningof the steel. By controlling the stability of the steel it is possibleto control the directionality thereof so that when these steels are coldworked up to 40%, it is possible to obtain a minimum compressive yieldstrength of K s.i. and a minimum tensile yield strength of K s.i., theselevels being achieved without incurring directionality to the steel suchthat the compressive yield strength to tensile yield strength ratio willexceed about .8.

It is to be noted from the data set forth in Table III that the steelsfalling within the scope of the subject invention which are cold workedfrom about 30% to 40% may, in some instances, exhibit what appears to bea ductility which in some applications may be considered to be on thelow side. While the amount of ductility itself is not objectionable,said design criteria may require that the steel have or exhibit a higherductility without seriously adversely affecting the level of themechanical properties as well as ductility. The ductility exhibitedbythe steel of the present invention can be improved by the applicationof a sub-critical anneal or a stress relief anneal which consists ofheating the steel to a temperature within the range between about 750 F.and 900 F. for a time period ranging between about 1 and about 16 hours.This stress relief anneal will have the effect ofincreasing theductility as measured by the percentage elongation without adverselyaffecting the ratio of the compressive yield'strength to the tensileyield strength. While some drop is noted in the attainable level of thetensile yield strength, the drop is relatively minor.

Heat FA-22, having the composition set forth in Table II, was coldworked to effect a reduction in the cross-sectional area of 30%. Thissteel exhibited a tensile yield strength of 208.6K s.i. and acompressive yield strength of 2044K s.i. or a ratio of compressive totensile yield strengths of 0.98. The steel also exhibited an elongationof 5.0%. Thereafter, the steel was annealed for 8 hours at a temperatureof about 800 F. After heat treatment as set forth hereinabove, thissteel exhibited a tensile yield strength of 189K s.i. and a compressiveyield strength of 202K s.i. Thus, it will be noted that the compressiveyield strength to tensile yield strength ratio is still greater than.8in this instance 1.08. It is also noted that the percentage elongationhas increased from 5% to 12% as a result of the stress relief anneal.Thus, it is clear that where higher ductilities are required, theapplication of a stress relief anneal within the temperature range andwithin the times set forth hereinbefore is effective for imparting therequisite ductility to these steels. These ductilit-ies are obtainedwithout seriously adversely afiecting the level of the mechanicalproperties and very little change is noted in the ratio of thecompressive yield strength to the tensile yield strength.

The steels of this invention having a composition within the range setforth in Table I are made in the wellknown manners which are common inthe stainless steel industry. No difficulty is encountered in hotworking the steels and standard rnill equipment is used to supply thecold reductions to the steels of this invention in the forms in whichthey are used in regular commercial products sold to the industry today.

We claim:

1. A work hardened stainless steel cold reduced between about and about40% and having a composition consisting essentially of from about 0.08%to about 0.12% carbon, up to about 1.25% manganese, up to about 1%silicon, from about 15.3% to about 16.6% chromium, from about 5.5 toabout 6.2% nickel, from about 0.025% to about 0.06% nitrogen, and thebalance essentially iron with incidental impurities, said stainlesssteel being characterized by exhibiting a ratio of compressive yieldstrength to tensile yield strength each measured in the longitudinaldirection of greater than about 0.8.

2. A work hardened stainless steel cold reduced between about and andhaving a composition essentially consisting of from about 0.08% to about0.12% carbon, from about 0.5% to about 1.25% manganese, up to about 1%silicon, from about 15.3% to about 16.6% chromium, from about 5.5% toabout 6.2% nickel, from about 0.025 to about 0.06% nitrogen, and thebalance essentially iron with incidental impurities, said stainlesssteel being characterized by exhibiting a minimum longitudinal tensileyield strength of 180K s.i. and a minimum longitudinal compressive yieldstrength of 160K s.i.

3. A work hardened stainless steel cold reduced between about 10% andabout 40% and having a composition consisting essentially of about 0.1%carbon from about 0.8% to about 1.0% manganese, from about 0.5% to about0.75% silicon, from about 15.9% to about 16.4% chromium, from about 5.8%to about 6.0% nickel, from about .03% to about 104%. nitrogen, and thebalance essentially iron with incidental impurities, said stainlesssteel being characterized by exhibiting a ratio of compressive yieldstrength to tensile yield strength each measured in the longitudinaldirection of greater than about 0.8.

4. Work hardened stainless steel sheets, strips, wire, bars and platescold reduced between 10% and 40% and having a composition essentiallyconsisting of from about 0.08% to about 0.12% carbon, up to about 1.25%manganese, up to about 1% silicon, from about 15.3% to about 16.6%chromium, from about 5.5% to about 6.2% nickel, from about 0.025% toabout 0.06% nitrogen, and the balance essentially iron with incidentalimpurities, said stainless steel being characterized by exhibiting aratio of compressive yield strength to tensile yield strength eachmeasured in the longitudinal direction of greater than about 0.8.

5. A work hardened stainless steel article of manufacture characterizedby exhibiting a ratio of longitudinal compressive yield strength tolongitudinal tensile yield strength of greater than about 0.8 after thestainless steel from which said article is formed has been cold workedsufiicient to effect a reduction in the cross sectional area of thesteel of between 10% and 40%, a ductility as measured by the percentelongation of at least 10% after the work hardened stainless steel hasbeen stress relief annealed at a temperature within the range of about750 F. to about 900 F. and a composition consisting essentially of fromabout 0.08% to about 0.12% carbon, up to about 1.25% manganese, up toabout 1% silicon, from about 15.3% to about 16.6% chromium, from about5.5% to about 6.2% nickel, from about 0.025 to about 0.06% nitrogen, andthe balance essentially iron with incidental impurities.

6. A Work hardened stainless steel article of manufacture characterizedin that the stainless steel from which said article is formed exhibits aminimum longitudinal tensile yield strength of about 180K s.i. and aminimum longitudinal compressive yield strength of about 160K s.i. afterthe stainless steel has been cold worked sufiicient- 1y to effect areduction in the cross sectional area of between about 30% and about40%, and a composition consisting essentially of from about 0.08% toabout 0.12%

carbon, up .to about 1.25% manganese, up to about 1% silicon, from about15.3% to about 16.6% chromium, from about 5.5 to about 6.2% nickel, fromabout 0.025 to about 0.06% nitrogen, and the balance essentially ironwith incidental impurities.

7. A work hardened stainless steel article of manufacture characterizedby exhibiting a ratio of longitudinal compressive yield strength tolongitudinal tensile yield strength of greater than about 0.8 after thestainless steel from which said article is formed has been cold Workedsufiicient to effect a reduction in the cross sectional area of thesteel of between 10% and 40% a ductility as measured by the percentelongation of at least 10% after the work hardened stainless steel hasbeen stress relief anealed at a temperature in the range of about 750 F.to about 900 F., and a composition consisting essentially of about 0.1%carbon, from about 0.8% to about 1.0% manganese, from about 0.5% toabout 0.75% silicon, from about 15.9% to about 16.4% chromium, fromabout 5.8% to about 6.0% nickel, from about .03% to about .04% nitrogen,and the balance essentially iron with incidental impurities.

8. A work hardened stainless steel cold reduced between about 10% andabout 40%, stress relief annealed at a temperature within the rangebetween about 0 F. and about 900 F., and having a composition consistingessentially of from about 0.08% to about 0.12% carbon, up to about 1.25%manganese, up to about 1% silicon, from about 15.3% to about 16.6%chromium, from about 5.5 to about 6.2% nickel, from about 0.025 to 0.06%nitrogen, and the balance essentially iron with incidental impurities,said stainless steel being characterized by exhibiting a ratio ofcompressive yield strength to tensile yield strength each measured inthe longitudinal direction of greater than about 0.8, and a ductility asmeasured by the percent elongation of greater than about 10%.

9. A work hardened stainless steel cold reduced between about 10% andabout 40%, stress relief annealed at a temperature within the rangebetween about 750 F. and about 900 F., and having a-compositionconsisting essentally of about 0.1% carbon, from about 0.8% to about1.0% manganese, from about 0.5 to about 0.75% silicon, from about 15.9%to about 16.4% chromium, from about 5.8% to about 6.0% nickel, fromabout 03% to about 04% nitrogen, and the balance essentially iron withincidental impurities, said stainless steel being characterized byexhibiting a ratio of compressive yield strength each measured in thelongitudinal direction of greater than about 0.8, and a ductility asmeasured by the percent elongation of greater than about 10%.

10. A work hardened stainless steel cold reduced about 30% and having acomposition consisting essentially of about 0.1% carbon, about 0.9%manganese, about 0.6% silicon, about 16.4% chromium, about 6% nickel,about 0.03% nitrogen, and the balance essentially iron with incidentalimpurities, said stainless steel being characterized by exhibiting aratio of longitudinal compressive yield strength to longitudinal tensileyield strength of greater than about 0.8.

References Cited by the Examiner UNITED STATES PATENTS 1/1958 Waxweiler75-128.5 9/1959 Waxweiler 75128.5

OTHER REFERENCES DAVID L. RECK, Primary Examiner.

10. A WORK HARDENED STAINLESS STEEL COLD REDUCED ABOUT 30% AND HAVING ACOMPOSITION CONSISTING ESSENTIALLY OF ABOUT 0.1% CARBON, ABOUT 0.9%MANGANESE, ABOUT 0.6% SILICON, ABOUT 16.4% CHROMIUM, ABOUT 6% NICKEL,ABOUT 0.03% NITROGEN, AND THE BALANCE ESSENTIALLY IRON WITH INCIDENTALIMPURITIES, SAID STAINLESS STEEL BEING CHARACTERIZED BY EXHIBITING ARATIO OF LONGITUDINAL COMPRESSIVE YIELD STRENGTH TO LONGITUDINAL TENSILEYIELD STRENGTH OF GREATER THAN ABOUT 0.8.